Finnish troops had occupied some of the small islands and used them as observation points. One of them was Bengtskär. Bengtskär was occupied by a platoon-sized force under command of Lt. Fred Luther. The nearest support was gunboats Uusimaa and Hämeenmaa that were stationed west from Bengtskär and they had about two hours travel. Soviet command decided to eliminate this island by using a small commando attack on 25/26. July. The attack force consisted of two 30-men attack platoons and a demolition group in four 56-ton MO-boats. The boats sailed silently to the pier soon after one o'clock and the attackers effectively eliminated the lone guard and approached the buildings. When the defenders were alerted, it was almost too late to arrange any defence. The Finns had to seek protection in the upper levels of the lighthouse building. Soviet MO-boats fired at the building windows. Finns directed own coastal battery fire from Örö and Granholma islands (both about 7 miles away from Bengtskär) by radio, first to the boats and then to enemy troops on the island itself. Command was taken by 3rd class petty officer Bjelke after Lt. Luther had been wounded second time.
At three o'clock the situation of the defenders was hopeless, but rescue was coming. The smoke-belching gunboats arrived and chased enemy boats away. Then they opened fire against enemy troops on the island and sent a machine gun squadron to the island. Soviet fighter planes attacked gunboats and Finnish planes from Turku attacked in turn Soviet planes. It was soon clear that the demolition attempt was failed. At about six o'clock MO-boats approached the island and tried to evacuate remaining attackers. Enemy boat PK 238 almost reached the island but was sunk by Uusimaa. Patrol boat VMV 13 saved 16 survivors. One hour later Finnish coastal jaegers arrived and landed immediately. Defenders in the lighthouse were in good position to point out enemies. At 09.15 the lighthouse building was cleared and wounded defenders evacuated. Close combat continued till 18.45 and all 13 captured enemies were wounded. Several Soviet soldiers committed suicide with hand grenade.
The enemy had lost 40 men on the island, drowned and losses in MO-boats are unknown. The defenders of Bengtskär lost 16 men and Navy vessels four. On 27 July the lighthouse building was hit by a bomb that landed in ground floor that was full of wounded men waiting to be evacuated. At least 11 men were killed.